History

Cryogenics, a branch of low-temperature physics concerned with the effects of very low temperatures on the phenomenas and materials, was first investigatred by Micahel Faraday, who demonstrated that gases could be liquefied leading to the production of low temperatures around 173 K.

The most prominent contributors to cryogenic technology include five distinguished physicist and chemists, namely, Andrews, C. de la Tour, Faraday, Joule and Thomson. Between 1820 and 1870, the latter two demonstrated the dependence of the gas's energy on the operating pressure and temperature. Andrew performed a series of experiments in 1869 with CO2 and discovered a critical temperature above which the liquid state cannot exists regardless of pressure. Later researcher studied the properties of permanent gases. A french scientist, Cailletet, liquified oxygen for the first time in 1877; Polish physicists Wroblewski and Olszewski liquified oxygen and air in large quantities during the 1880s; and English physicist, Dewar, first liquified hydrogen gas in 1898; and a Dutch physicist, Kamerlingh-Onnes, first liquified helium gas in 1908.

The Stirling brothers in Scothland developed a sophisticated hot-air engine during the years 1825 to 1840 while Faraday was busy with experiments to liquefy gases. When Stirling engine reversed and was used as a heat engine, it led to other important cryogenic developments. Scottish scientist Kirk developed an impressive chiller for cold storage and ice making during the early 1860s by adapting the Stirling cycle for refrigeration. Phillips scientists, Kohler and others improved the performance of the Kirk chiller and developed an efficient air liquifier between 1948 and 1954, which provided the refrigeration capability in the 20K range. The Kirk cycle was later modified by Gifford and McMahon (G-M) in 1959 using a remotely located system composed of a compressor, regenerator and expansion device. The significant difference in the G-M variation of the Stirling cycle lies in the complete seperation of the compression unit from the expansion device.

Cryogens

The production of inert gases accelerated right after World War II due to the heavy demand for scientific research and defence applications. UNOCAL Corp. of Utah first started the production of gases for industrial and commercial applications. The company has a gas treatment plant, and a helium (He) purification and liquefaction plant that have been in operation since 1991 with liquefied helium shipping capacity. The most important commercial application of cryogenic gas liquefaction techniques is the storage and transportation of liquefied natural gas (LNG), a mixture largely composed of methane, ethane, and other combustible gases. Natural gas is liquefied at 110 K, causing it to contract to 1/600th of its volume at room temperature and making it sufficiently compact for swift transport in specially insulated tankers.

Liquid Nitrogen (LN2)

Liquid nitrogen is a farly inert gas medium and has unique properties that make it a most economical cryorefrigerant, which offers more than 40 times more refrigerating capacity per unit volume than liquid helium and more than 3 times of liquid hydrogen. The following three distinct developments promise a great demand for LN2, where exotic and large-volume commercial applications are involved:

Freezing of baked goods, shrimp, TV dinners, meats, soups, and so on requires a process known as CryoQuick in the food industry. Refrigeration for trucks, trailers and railroad carts for in-transit preservation of fruits, vegetables, meats and other perishable food items requires a process known as CryoGuard. Deflating of molded rubber parts requires a process known as CryoTrim.

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Liquid Heium (LHe)

In the field of cryogenics, helium is utilized for a variety of reasons. Liquid helium has been used as a cryogenic refrigerant for various applications such as the particle accelerators, magnetic resonance imaging (MRI), nuclear magnetic resonance (NMR) and some experiments in physics where large magnetic field are required. The combination of helium's exteremely low molecular weight and weak, interatomic reactions yield interesting properties when helium is cooled below its critical temperature of 5.2 K to form a liquid.

Liquid Hydrogen

Liquid hydrogen has been widely used in applied cryogenics because of its minumum cost when produced in large volumes. However, its adverse chemical effects and stringent handling requirements largely offset this potential. It is widely used as a propellant in rockets, boosters, missiles and space vehicles.

Cryogenic Safety

Most cryogenic liquids are odorless, colorless, and tasteless when vaporized. When cryogenic liquids are exposed to the atmosphere, the cold boil-off gases condense the moisture in the air, creating a highly visible fog.

• Cryogenic liquids MUST be used in a well ventilated area. All crogenic liquids produce large volumes of gas when they vaporize. For example, one liter of liquid nitrogen dispalces 694 liters of air when it vaporizes. • When used in sealed containers, this vaporization can produce enourmous pressures. • Always wear proper gloves.• Always use proper containers designed for the transport and use of cryogenic liquids. • Examine containers and pressure relief valves for signs of defect. Never use a container which has defects.• Always handle these liquids carefully to avoid skin burns and frostbite. Exposure that may be too brief to affect the skin of the face or hands may damage delicate tissues, such as the eyes.• Boiling and splashing always occur when charging or filling a warm container with cryogenic liquid or when inserting objects into these liquids. Perform these tasks slowly to minimize boiling and splashing. Use tongs to withdraw objects immersed in a cryogenic liquid.• When transferring into a secondary container, do not fill the secondary container to more than 80% of capacity• Use wooden or rubber tongs to remove small items from cryogenic liquid baths. Cryogenic gloves are for indirect or splash protection only, they are not designed to protect against immersion into cryogenic liquids.

Critical Aspects

The cryogenic temperature range has been defined as from −150 °C (−238 °F) to absolute zero (−273 °C or −460 °F), the temperature at which molecular motion comes as close as theoretically possible to ceasing completely. Cryogenic temperatures are usually described in the absolute or Kelvin scale, in which absolute zero is written as 0 K, without a degree sign.

Cryogenic temperatures are considerably lower than those encountered in ordinary physical processes. At these extreme conditions, such properties of materials as strength, thermal conductivity, ductility, and electrical resistance are altered in ways of both theoretical and commercial importance. Because heat is created by the random motion of molecules, materials at cryogenic temperatures are as close to a static and highly ordered state as is possible. Temperatures below 3 K are primarily used for laboratory work, particularly research into the properties of helium. Helium liquefies at 4.2 K, becoming what is known as helium I. At 2.19 K, however, it abruptly becomes helium II, a liquid with such low viscosity that it can literally crawl up the side of a glass and flow through microscopic holes too small to permit the passage of ordinary liquids, including helium I. (Helium I and helium II are, of course, chemically identical.) This property is known as superfluidity.